As a method for detecting a defect of a circuit pattern formed on a wafer by comparing images in a manufacturing process of a semiconductor device, for example, a pattern comparing inspection method is disclosed in JP-A No. 258703/1993. The method uses an SEM in which, a point focused electron beam is scanned. The SEM type inspection apparatus is higher in resolution than optical inspection systems and it has a feature for enabling such an electrical defect as a connecting failure to be detected. However, because the SEM type inspection apparatus scans an electron beam on a specimen surface two-dimensionally to obtain an image, the scanning time is long. This disadvantage will become a substantial obstacle for reducing the inspection time.
As an electron beam inspection method that has successfully reduced the inspection time, for example, the JP-A No. 249393/1995 discloses a projection type inspection apparatus, which illuminates a rectangular electron beam onto semiconductor wafer and forms an image of buck scattering and secondary electrons with use of electron lenses. The projection type inspection apparatus can illuminate an electron beam with a larger current than that of the SEM type at a time, thereby obtaining a plurality of images collectively. The projection type is thus expected to form images more quickly than those of the SEM type, that is, the scanning electron type.
On the other hand, a secondary electron mapping type inspection apparatus cannot obtain a sufficient resolution due to the aberration of the objective lens, thereby it is difficult to obtain a required defect detection sensitivity. The JP-A No. 108864/1999 points out such disadvantages of the apparatus. The JP-A No. 108864/1999 discloses a mirror electron imaging type wafer inspection apparatus that uses electrons pulled back (hereunder, to be referred to as mirror electrons or mirror reflecting electrons) before colliding with a specimen due to a negative electric field formed just above the wafer as imaging electrons.
Here, the mirror electron imaging type wafer inspection apparatus will be described. The mirror electron imaging type wafer inspection apparatus obtains an image to be used for inspection with use of a mirror electron microscope. An inspection image is obtained by illuminating an electron beam onto a specimen and forms an image of the reflecting electron beam. At this time, a negative potential is applied onto the surface of the specimen in advance so that the illuminated electron beam is reflected on a specific equipotential surface in the vicinity of the specimen surface without reaching the specimen surface. The electrons reflected on an equipotential surface in the vicinity of the specimen surface such way are referred to as “mirror electrons”. Because the equipotential surface of the specimen surface is influenced by the information of an unevenness and a potential change of the specimen surface itself, the image to be obtained is also influenced by the information of the unevenness and the potential change of the specimen surface when the mirror electrons are imaged. Consequently, shape and electrical defects of the specimen surface can be detected by comparing such a mirror electron image with a reference image, respectively.